Thermosensitive P(AAc‐co‐NIPAm) Hydrogels Display Enhanced Toughness and Self‐Healing via Ion–Ligand Interactions

Abstract

Hydrogels containing thermosensitive polymers such as poly(N‐isopropylacrylamide) (P(NIPAm)) may contract during heating and show great promise in fields ranging from soft robotics to thermosensitive biosensors. However, these gels often exhibit low stiffness, tensile strength, and mechanical toughness, limiting their applicability. Through copolymerization of P(NIPAm) with poly(Acrylic acid) (P(AAc)) and introduction of ferric ions (Fe3+) that coordinate with functional groups along the P(AAc) chains, here a thermoresponsive hydrogel with enhanced mechanical extensibility, strength, and toughness is introduced. Using both experimentation and constitutive modeling, it is found that increasing the ratio of m(AAc):m(NIPAm) in the prepolymer decreases strength and toughness but improves extensibility. In contrast, increasing Fe3+ concentration generally improves strength and toughness with little decrease in extensibility. Due to reversible coordination of the Fe3+ bonds, these gels display excellent recovery of mechanical strength during cyclic loading and self‐healing ability. While thermosensitive contraction imbued by the underlying P(NIPAm) decreases slightly with increased Fe3+ concentration, the temperature transition range is widened and shifted upward toward that of human body temperature (between 30 and 40 °C), perhaps rendering these gels suitable as in vivo biosensors. Finally, these gels display excellent adsorptive properties with a variety of materials, rendering them possible candidates in adhesive applications.

l-Citrulline-Modified Ti3C2Tx MXene Nanosheets Embedded in Polyacrylamide/Sodium Alginate Hydrogels for Electromagnetic Interference Shielding

“…These results indicated that the tensile strength decreased with increasing FeCl 2 concentration, and the elongation at break increased with increasing FeCl 2 concentration. The main reason for this phenomenon was that FeCl 2 acted as a bridge in the composite hydrogel, which could enhance the ductility; 35 and Fe 2+ addition also resulted in increasing water content and formation of more hydrogen bonds, which provided an effective energy dissipation path and enhanced the elongation at break. 36,37 The variation in the EMI shielding efficiency as a function of C-MXene nanosheet and FeCl 2 concentrations for the resultant hydrogels are presented in Figure 5.…”

Coupled bond dynamics alters relaxation in polymers with multiple intrinsic dissociation rates

“…The simplicity and robustness of this approach, combined with its compatibility with other first-order physical phenomena, may render it useful for predictive design of diverse hybrid networks including elastomers with charge interactions, 16,65,66 vitrimers, 6 and metallopolymers. 7,9 Therefore, in future work, this method may be utilized in conjunction with additional modeling approaches for other significant phenomena (e.g., entanglement, reptation, etc.) to investigate polymer micromechanics or guide design applications.…”

“…To amend the rule of mixture we posit that some fraction, x, of stable bonds in hybrid networks relax at rate k d due to reconfiguration of adjacent dynamic bonds, whereas the fraction 1 À x cannot relax over intermediate timescales because they are constrained by the stable bond network structure, suffer from relaxation retardation due to bond lifetime renormalization effects, 33,35 or are otherwise constrained by steric interactions (as in the case of the 2D, highly packed networks observed in the current discrete model). Incorporating x into eqn (9), gives a coupled rule of mixture for hybrid networks:…”

“…within the same continuous network. [6][7][8][9][10][11][12] In such systems, the stable bonds often form a scaffold that supports the dynamic bonds throughout the material. Under these conditions, the stable bonds may preserve suitably high moduli, while incorporation of the sacrificial or reversible dynamic bonds introduces tunable viscoelasticity 6,[13][14][15][16][17] and self-healing ability.…”

“…Under these conditions, the stable bonds may preserve suitably high moduli, while incorporation of the sacrificial or reversible dynamic bonds introduces tunable viscoelasticity 6,[13][14][15][16][17] and self-healing ability. 9,18,19 While designing such materials, researchers often employ physically motivated constitutive modeling techniques through which the properties of individual bonds may be used to predict the globally emergent responses of the networks. [20][21][22][23] However, real-time experimental characterization of such materials' microstructures remains exceedingly challenging and is relatively limited to techniques such as small angle neutron scattering, 24 or inference from diffusion and rheology data.…”